The present invention is in the technical field of mass flow measurement of the constituent chemical species of a flowing fluid mixture. It relates to a system and a method for real-time sensing and recording of the mass flow rates of the individual components of interest of a fluid mixture flowing in a pipe or other conduit. More specifically, the invention relates to a multi-species mass flow rate measurement system which can be inserted in a fluid stream to provide output signals indicative of the fluid species mass flow rates and a method of determining the mass flow rates of each of the detected species of interest from the species concentration detector and bulk fluid stream flow rate detector comprising the system. The invention has broad applicability to industrial processes and the measurement of pollutants in vehicle exhaust gas.
It is desirable to accurately measure the mass flow rates of the constituent species of fluid streams in many industrial processes and for environmental compliance testing purposes. But it is well-known that under dynamic conditions in which the concentration of species of interest and the fluid flow rate are both changing, there can be a significant error in determining the mass flow rates of the species in the usual manner involving the use of two different devices or probes—one for measuring the concentration of the species of interest and a separate probe, located upstream or downstream from the first probe for determining the bulk fluid flow rate. The error results from a variable time delay due to the inability to continuously synchronize the concentration data series in transient fluid flow with the bulk fluid flow rate data series when the probes are placed apart from each other and from the disturbance of the flow rate signal when the concentration probe is placed in close proximity to the flow measurement probe. Thus there exists the need for an improved mass flow measurement device which is accurate under transient flow conditions in which the concentration of the species of interest is also changing rapidly.
For example, it has been desirable to measure the exhaust gas mass flow rates from internal combustion engines in a laboratory testing setting since the United States Environmental Protection Agency (EPA) began regulating automobile emissions in the 1970s. More recently it has become desirable to measure the exhaust emissions mass flow rates on moving vehicles operated in the real world for regulatory compliance testing purposes [Breton, also the current inventor, U.S. Pat. Nos. 6,148,656; 6,382,014 B1; 6,470,732 B1, henceforth referred to collectively as Breton].
There is also a desire to minimize the effects, e.g. the backpressure created by the measurement devices, on the flow being measured and on the machine or process associated with the flow being measured. Therefore, there is a need for the invention to be practiced in a compact geometrical footprint to minimize the disturbance to the bulk flow, thereby minimizing the effect of the measurement device on the system or machine for which the fluid flow is being measured. For this reason, Breton teaches a mobile emissions measurement system using an exhaust gas flow rate measurement module employing an averaging pitot tube with a narrow cross section. The module is comprised of the pitot tube, a separate pressure sensing means, a separate temperature sensing means, and a gas concentration analyzer sampling port, all permanently mounted in a movable module so that whenever the module is connected to a vehicle tailpipe, it inherently incorporates the necessary upstream and downstream flow diameters needed for accurate flow sensing, even for the worst-case installation expected.
Traditional EPA exhaust emissions compliance testing employs gas and particulate matter analyzers based on specific measurement principles, depending on the chemical species of interest. Because those measurement principles are used by EPA when regulatory compliance or certification testing is done by the Agency, engine and vehicle manufacturers typically use those same analyzers or analyzer technologies to ensure consistency with the regulators' testing results, i.e. there are de facto standard analyzer technologies. For this reason Breton teaches a “Real-Time On-Road Vehicle Exhaust Gas Modular Flowmeter and Emissions Reporting System” in which the exhaust gas analyzers employ the same measurement principles referenced by EPA regulations for each chemical species being measured.
Using the de facto standard analyzer technologies can have a number of drawbacks however, and those drawbacks can be significant for many potential users of a mobile measurement system. For example, the de facto standard gas analyzers must be “warmed up” before use. To ensure accuracy, the standard analyzers must also be calibrated on a regular basis, and “zeroed” and “spanned” prior to each test, requiring the use of numerous, expensive calibration gases of various known concentrations, each contained in a large gas cylinder. Because of these requirements, calibration gases often need to be carried with vehicles being tested.
The traditional analyzers must also be mounted onto or inside the vehicle being tested, requiring a significant volume of space in the vehicle. And some vehicles don't have suitable space for mounting.
Because the concentration signals from remotely mounted gas analyzers are significantly delayed in time compared with the flow rate signals from the flow meter, it is necessary to measure the relative time delay initially and whenever the length of the gas sample hose is changed. Repairs are often necessary because the traditional analyzers draw a sample from the exhaust stream using a pump, check valves, and other plumbing prone to failures.
Parks [U.S. Pat. Nos. 9,000,374 and 9,068,933] teaches a laser-based Exhaust Gas Recirculation (EGR) diagnostic probe for engine research. The probe is capable of measuring the concentration of some gas species. Other chemical species could also be measured by adding additional laser light frequencies to the system. This type of species concentration analyzer with the appropriate frequencies of operation would be desirable for at least one embodiment of the present invention related to exhaust gas mass flow measurement.
Even though Breton teaches how to minimize measurement error by fixing the gas analyzer sampling probe tip in close proximity to the flow sensing means, it is not possible to have them arbitrarily close to each other due to potential disturbance of the exhaust flow profile near the flow sensing means, by the presence of the sampling probe. So the overall accuracy of the traditional measurement system is reduced by a small, variable time shift between the flow rate signal and the gas concentration signal as the engine speed and load change quickly. Breton previously proved that an averaging pitot tube can be highly accurate over large exhaust flow turndown ratios, i.e. large changes in flow rates, and is well suited for vehicle emissions testing because it can be employed with a small geometrical cross-sectional area, minimizing backpressure in the flow to be measured. For testing applications requiring insertion into, or attachment onto a vehicle's tailpipe, the cross-sectional area to flow must be kept as small as possible to prevent the creation of significant backpressure which could alter the operation of the vehicle's engine, thereby affecting the test results. Some vehicles are extremely sensitive to exhaust backpressure because it can interfere with their engine control systems. The minimum averaging pitot tube cross-sectional sizes available commercially are determined by their ability to avoid bending in moving flow streams and their ability to resist process vibrations. It is desirable to avoid using cross-sectional sizes greater than these minimum sizes for exhaust gas measurements on vehicles with narrower tailpipes. Therefore, any alterations of the standard pitot tube probe designs which result in a larger cross-section are undesirable.
It would also be advantageous to possess the ability to continuously measure the exhaust gas mass flow rates of the pollutants of moving vehicles operated in the real world with an embedded measurement system low enough in cost to be able to be permanently installed by vehicle manufacturers on all new vehicles for providing input to a vehicle's on-board diagnostics (OBD) system to indicate the health of the emissions controls, the fuel economy of the vehicle, and for the continuous measurement of the regulatory compliance level of the vehicle over its lifetime. Unfortunately, there are significant manufacturing challenges to making compact devices as described above because increased functionality from a single probe would normally necessitate the use of a larger probe which is disadvantageous because of increased resistance to fluid flow, hence increased backpressure. As stated above, increased exhaust backpressure on a vehicle can alter the vehicle's operation and lead to erroneous test results. Therefore, it is desirable to increase the measurement capabilities of a new species mass flowmeter device that is no larger in cross-section than current, minimum size averaging pitot tubes, while also increasing overall accuracy and ease of use of a larger system based on such devices.
The present invention teaches a mass flow measurement system for constituent gas species in a contained gas flow which eliminates all of the aforementioned drawbacks, thereby increasing the ease of use of the measurement system for the testing of vehicle emissions, as well as the accuracy in measuring fluid species mass flow rates for transient flows and the simplicity and low cost of a mass flow measurement device for high volume applications such as automobiles or numerous industrial applications.